Adsorption processes for the separation of oxygen and nitrogen from air are being increasingly used for commercial purposes for the last two decades. Oxygen requirements in sewage treatment, fermentation, cutting and welding, fish breeding, electric furnaces, pulp bleaching, glass blowing, medical purposes and in the steel industries particularly when the required oxygen purity is 90 to 94% is being largely met by adsorption based pressure swing or vacuum swing processes. It is estimated that at present, 4-5% of the world's oxygen demand is met by adsorptive separation of air. However, the maximum attainable oxygen purity by adsorption processes is around 95% with separation of 0.934 mole percent argon present in the air from oxygen being a limiting factor to achieve 100% oxygen purity. Further-more, the adsorption based production of oxygen from air is economically not competitive to cryogenic fractionation of air for production levels of more than 100 tonnes oxygen per day. Of the total cost of oxygen production by adsorption processes, it is estimated that capital cost of equipment and power consumption are the two major factors influencing the overall cost with their share being 50% and 40% respectively. Besides the other factors like process and system designs, the adsorbent is the key component which can bring down the cost of oxygen production by adsorption. The adsorbent selectivity and capacity are important parameters for determining the size of adsorption vessels, compressors or vacuum pumps. It is desirable to have an adsorbent which shows a high adsorption capacity as well as selectivity for nitrogen compared to oxygen. The improvement in these properties of the adsorbent directly results in lowering the adsorbent inventory of a system along with the size and power consumption of the air compressor or vacuum pump. Furthermore, adsorbent having a high nitrogen adsorption selectivity and capacity can also be used to produce reasonably pure nitrogen along with oxygen by evacuating nitrogen adsorbed on the adsorbent.
It is therefore, highly desirable, if not absolutely essential, for an adsorbent to have a good adsorption capacity and adsorption selectivity for a particular component sought to be separated.
Adsorption capacity of the adsorbent is defined as the amount in terms of volume or weight of the desired component adsorbed per unit volume or weight of the adsorbent. The higher the adsorbent's capacity for the desired components the better is the adsorbent as the increased adsorption capacity of a particular adsorbent helps to reduce the amount of adsorbent required to separate a specific amount of a component from a mixture of particular concentration. Such a reduction in adsorbent quantity in a specific adsorption process brings down the cost of a separation process.
Adsorption selectivity of component A over B is defined as EQU .alpha.A/B=X.sub.A Y.sub.B /Y.sub.A X.sub.B
where .varies. is the adsorption selectivity, X is the adsorbed concentration and Y is gas-phase concentration respectively. The expression gas-phase concentraction means the amount of unadsorbed component remaining in the gas-phase. The adsorption selectivity of a component depends on
steric factors such as differences in the shape and size of the adsorbate molecules;
equilibrium effect, i.e. when the adsorption isotherms of the component of the gas mixture differ appreciably;
kinetic effect, when the components have substantially different adsorption rates.
It is generally observed that for a process to be commercially economical, the minimum acceptable adsorption selectivity for the desired component is about 3 and when the adsorption selectivity is less than 2, it is difficult to design an efficient separation process.
In the prior art, adsorbents which are selective for nitrogen from its mixture with oxygen and argon have been reported wherein the zeolites of type A, X and mordenite have been used after ion exchanging with alkali and/or alkaline earth metal ions. However, the adsorption selectivity reported for the commercially used adsorbents for this purpose varies from around 3 to 5. The efforts to enhance the adsorption capacity and selectivity has been reported by increasing the number of exchangeable cations into the zeolite structure by modifying the chemical composition of the zeolite (Reference Coe Si/Al ratio). The adsorption selectivity for nitrogen has also been substantially enhanced by exchanging the zeolite with cations like lithium and/or calcium in some zeolite types. In the present invention, we report a new chemical composition using faujasite type zeolite having alkali, alkaline or rare earth metal ions which give substantially high nitrogen adsorption capacity and selectivity compared to commercially employed adsorbents for oxygen production from air.
Zeolites which are microporous crystalline aluminosilicates are finding increased applications as adsorbents for separating mixtures of closely related compounds. Zeolites have a three dimensional network of basic structural units consisting SiO.sub.4 and AlO.sub.4 tetrahedral linked to each other by sharing of apical oxygen atoms. Silicon and aluminum atoms lie at the center of the tetrahedral. The resulting aluminosilicate structure which is generally highly porous possesses three dimensional pores the access to which is through molecular sized windows. In a hydrated form, the preferred zeolites are generally represented by the following Formula [I] EQU M.sub.2/n O:Al.sub.2 O.sub.3 :xSiO.sub.2 :wH.sub.2 O (I)
where "M" is a cation which balances the electrovalence of the tetrahedral and is generally referred to as extra framework exchangeable cation, n represents the valency of the cation, x and w represent the moles of SiO.sub.2 and water respectively. The cations may be any one of the number of cations which will hereinafter be described in detail.
The attributes which make the zeolites attractive for separation include, an unusually high thermal and hydrothermal stability, uniform pore structure, easy pore aperture modification and substantial adsorption capacity even at low adsorbate pressures. Furthermore, zeolites can be produced synthetically under relatively moderate hydrothermal conditions.
Zeolite of type X structure as described and defined in U.S. Pat. No. 2,882,244 are the preferred adsorbents for adsorption separation of the gaseous mixture described in this invention. Zeolite of type X in hydrated or partially hydrated form can be described in terms of metal oxide of the Formula II. EQU (0.9+/-0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5+/-0.5)SiO.sub.2 :wH.sub.2 O(II)
where "M" represents at least one cation having valence n, w represents the number of moles of water the value of which depends on the degree of hydration of the zeolite. Normally, the zeolite when synthesized has sodium as exchangeable cations.
Powdered zeolites as such have very little cohesion and it is, therefore, necessary to use appropriate binders to produce the adsorbent in the form of particles such as extrudates, aggregates, spheres or granules to suit commercial applications. Zeolitic content of the adsorbent particle vary from 60 wt % to 98 wt % depending on the type of binder used. Clays such as bentonite, kaolin, and attapulgite are normally used inorganic binders for agglomeration of zeolite powders.